1. Field of the Invention
The present invention relates to radio communications, and in particular, to handover in a shared radio access network environment.
2. Related Art and Other Considerations
In a typical cellular radio system, mobile user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The User Equipment (UE) is the mobile terminal by which a subscriber can access services offered by the operator's Core Network (CN). The RAN (Radio Access Network) is the part of the network that is responsible for the radio transmission and control of the radio connection. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
FIG. 7 and the explanation below introduce the assumed network architecture and context for describing example embodiments of the present invention. The RNS (Radio Network Subsystem) controls a number of Base Stations in the radio access network. The RNC (Radio Network Controller) controls radio resources and radio connectivity within a set of cells. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station (BS). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell.
The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). In FIG. 7, each base station is shown with three representative cells. The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UMTS is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a radio access network providing wideband code division multiple access (WCDMA) to user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM-based radio access network technologies.
As those skilled in the art appreciate, in W-CDMA technology a common frequency band allows simultaneous communication between a user equipment unit (UE) and plural base stations. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudo-noise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the user equipment units (UEs). Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately demodulated at a receiving station. The high speed PN modulation also allows the receiving station to advantageously generate a received signal from a single transmitting station by combining several distinct propagation paths of the transmitted signal. In CDMA, therefore, a user equipment unit (UE) need not switch frequency when handoff of a connection is made from one cell to another. As a result, a destination cell can support a connection to a user equipment unit (UE) at the same time the origination cell continues to service the connection. Since the user equipment unit (UE) is always communicating through at least one cell during handover, there is no disruption to the call. Hence, the term “soft handover.” In contrast to hard handover, soft handover is a “make-before-break” switching operation.
The Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN) accommodates both circuit switched and packet switched connections. In this regard, in UTRAN the circuit switched connections involve a radio network controller (RNC) communicating with a mobile switching center (MSC), which in turn is connected to a connection-oriented, external core network, which may be (for example) the Public Switched Telephone Network (PSTN) and/or the Integrated Services Digital Network (ISDN). On the other hand, in UTRAN the packet switched connections involve the radio network controller communicating with a Serving GPRS Support Node (SGSN) which in turn is connected through a backbone network and a Gateway GPRS support node (GGSN) to packet-switched networks (e.g., the Internet, X.25 external networks). MSCs and GSNs are in contact with a Home Location Register (HRL), which is a database of subscriber information.
There are several interfaces of interest in the UTRAN. The interface between the radio network controllers (RNCs) and the core network(s) is termed the “Iu” interface. The interface between a radio network controller (RNC) and its base stations (BSs) is termed the “Iub” interface. The interface between the user equipment unit (UE) and the base stations is known as the “air interface” or the “radio interface” or “Uu interface”. In some instances, a connection involves both a Serving or Source RNC (SRNC) and a target or drift RNC (DRNC), with the SRNC controlling the connection but with one or more diversity legs of the connection being handling by the DRNC. An Inter-RNC transport link can be utilized for the transport of control and data signals between Source RNC and a Drift or Target RNC, and can be either a direct link or a logical link. An interface between radio network controllers (e.g., between a Serving RNC [SRNC] and a Drift RNC [DRNC]) is termed the “Iur” interface.
The radio network controller (RNC) controls the UTRAN. In fulfilling its control role, the RNC manages resources of the UTRAN. Such resources managed by the RNC include (among others) the downlink (DL) power transmitted by the base stations; the uplink (UL) interference perceived by the base stations; and the hardware situated at the base stations.
Those skilled in the art appreciate that, with respect to a certain RAN-UE connection, an RNC can either have the role of a serving RNC (SRNC) or the role of a drift RNC (DRNC). If an RNC is a serving RNC (SRNC), the RNC is in charge of the connection with the user equipment unit (UE), e.g., it has full control of the connection within the radio access network (RAN). A serving RNC (SRNC) is connected to the core network. On the other hand, if an RNC is a drift RNC (DRNC), its supports the serving RNC (SRNC) by supplying radio resources (within the cells controlled by the drift RNC (DRNC)) needed for a connection with the user equipment unit (UE). A system which includes the drift radio network controller (DRNC) and the base stations controlled over the Iub Interface by the drift radio network controller (DRNC) is herein referenced as a DRNC subsystem or DRNS. An RNC is said to be the Controlling RNC (CRNC) for the base stations connected to it by an Iub interface. This CRNC role is not UE specific. The CRNC is, among other things, responsible for handling radio resource management for the cells in the base stations connected to it by the Iub interface.
The UTRAN interfaces (Iu, Iur and Iub) have two planes, namely, a control plane (CP) and a user plane (UP). In order to control the UTRAN, the radio network application in the different nodes communicate by using the control plane protocols. The RANAP is a control plane protocol for the Iu interface; the RNSAP is a control plane protocol for the Iur interface; and NBAP is a control plane protocol for the Iub interface. The control plane protocols are transported over reliable signaling bearers. The transport of data received/transmitted on the radio interface occurs in the user plane (UP). In the user plane, the data is transported over unreliable transport bearers. The serving radio network controller (SRNC) is responsible for establishing the necessary transport bearers between the serving radio network controller (SRNC) and the drift radio network controller (DRNC).
The RAN decides the role of an RNC (SRNC or DRNC) when the UE-RAN connection is being established. Normally, the RNC that controls the cell where the connection to the UE is initially established is assigned the SRNC role for that UE connection. As the UE moves, the connection is maintained by establishing radio communication branches via new cells, possibly also involving cells controlled by other RNCs (i.e., DRNCs).
In FIG. 8, RNC 261 acts as SRNC for the connections to the single UE shown. In FIG. 9, RNC 261 acts as SRNC for the connections to the UE. The connection to UE is, after successive handovers, now communicated via a cell controlled by RNC 262, thus acting as DRNC for this connection. In FIG. 8 and FIG. 9, for the purpose of illustrating the RNC roles, only the SRNC has an interface to the CN. It should be understood that all RNCs have a CN interface.
For each mobile that the SRNC is serving, the SRNC stores a bit string which permanently identifies the mobile. According to the RAN system specified by 3GPP, this bit string is the IMSI, and is transferred to the SRNC from the CN using a Common ID procedure over the Iu interface at connection establishment. The “IMSI” is the international mobile subscriber identity (IMSI); “PLMN” refers to the public land mobile network (PLMN). The international mobile subscriber identity (IMSI) [which comprises not more than fifteen digits] comprises three components: a mobile country code (MCC)[three digits]; a mobile network code (MNC)[two or three digits]; and a mobile subscriber identification number (MSIN). The home-public land mobile network (HPLMN) id [HPLMNid] of the user equipment unit can be extracted from the international mobile subscriber identity (IMSI). In this regard, the HPLMNid of the user equipment unit is the mobile country code (MCC)+the mobile network code (MNC).
In the 3GPP approach, the structure of the IMSI is not recognized or used by the SRNC. It is only used to coordinate a paging from one CN domain with a connection that is ongoing for the other CN domain (matching two bit strings).
The DRNC stores cell information for all cells it controls and all neighboring cells. When a mobile is using a dedicated radio channel (3gpp cell_DCH state), the UTRAN transmits to the mobile a list of channels for which the mobile is to measure the signal strength of a transmission received on each of those channels associated with neighboring cells. The channel to measure for a neighboring WCDMA cell is identified both by frequency and code. The mobile measures the signal strengths of transmissions received from each of these neighbor cells, and reports the strongest ones, which become candidates for handover. All of the neighbor cell information is transferred from the DRNC to the SRNC when the SRNC sets up a radio link in a certain cell belonging to the DRNC.
In this situation, a problem will occur (as described below) if two or more network operators share part of a radio network in a certain area, but also have their own individual radio networks to serve mobiles leaving the coverage of the shared network. Consider these two example scenarios:                The shared network may be deployed in one region of the country, whereas in the rest of the country, the operators have their own networks.        The shared network covers the whole country with a certain radio access technology (e.g., WCDMA), whereas each operator has its own network of another radio access technology (e.g., GSM).        
In a special case, the shared network can be identical to one of the individual networks. In general, the shared network as well as the individual networks could support any of one or more radio access technologies, (e.g., 2G and/or 3G).
The problem and the solution(s) in accordance with the present invention are described in the following, non-limiting, example context:                Two operators A and B, each having an individual network (of any radio access technology)        the shared network is a WCDMA network.        
In a shared network, subscribers of either operator A and B, or of any operator that has a roaming agreement with any of the operators A and B, can use the same shared network. A problem occurs when that subscriber is leaving the coverage of the shared network requiring a handover to one of the two individual networks of operator A or B. A subscriber of operator A should normally be handed over to operator A's individual network, and similarly for operator B's subscribers. However, in the 3GPP approach, cells of both the individual network A and B need to be defined as neighbor cells to a certain shared network cell. The mobile is informed about all these cells and measures signal strengths from all of them. When reporting cell measurements to the RAN, the mobile may report as a candidate for handover a cell that belongs to the “wrong” network. In other words, when the mobile leaves the shared network area and enters into operator A's network, it may still report as a candidate, a cell in operator B's network. This degrades performance and may lead to lost calls.
What is needed, therefore, and an object of the present invention, is a technique to provide the mobile a list of neighbor cells adapted to that specific mobile's subscription.